1. Field of the Invention
The present invention relates to an integrated optical circuit. More specifically, the present invention relates to a device and a method enabling to align an optical fiber and a submicronic waveguide of an integrated optical circuit.
2. Discussion of the Related Art
Integrated optical circuits are being increasingly used in the field of telecommunications, especially for the transmission, processing, or storage of data. Integrated optical circuits may have many functions, such as multiplexing, demultiplexing, modulation, demodulation, spectral routing, amplification, accumulation, filtering, or a resonator function.
Integrated optical or optoelectronic circuits are generally formed in and on semiconductors wafers similar to those used in microelectronics. An integrated optical circuit comprises one or several elementary optical components processing one or several light beams, the light beams being conveyed between the elementary optical components by optical waveguides. An integrated optoelectronic circuit further comprises one or several electronic components.
The integration of an increasing number of functions on same chip requires a miniaturization of the optical components and of the associated waveguides. When the waveguides have dimensions below one micrometer, one can speak of submicronic or nanometric waveguides. Currently, such waveguides may have cross-section areas on the order of 0.5×0.2 μm2 for waves in the visible and close infrared field and transmit optical modes of similar dimensions.
For medium and long-distance transmissions, that is, from a few meters to several kilometers, optical fibers are the privileged optical transport means. An optical fiber usable in the visible and close infrared range transmits or guides an optical mode with a diameter ranging between 10 μm and a few tens of micrometers. It is accordingly necessary to use specific devices for coupling the optical fibers with the submicronic waveguides so that the light beams can travel correctly between these structures guiding optical modes of different dimensions.
FIG. 1 illustrates a known coupling grating device between an optical fiber and a submicronic waveguide of an integrated optical circuit. Of course, this coupling device only takes up a small portion of an integrated circuit chip. Above a support 1, for example, made of silicon, is formed a submicronic waveguide comprising a core 3 surrounded with lower and upper layers 5 and 7 of different indexes forming an optical cladding. Lower and upper layers 5 and 7 are for example made of silicon oxide and core 3 is for example made of silicon. The optical index difference between the materials of the core and of the cladding of the waveguide enables to confine light beams within the core of waveguide 3. This waveguide extends towards integrated optical circuits, not shown.
A diffraction grating 9 is formed at the surface of core 3. Diffraction grating 9 is for example formed of an assembly of parallel grooves. It may be provided, as shown, to widen the submicronic waveguide at the level of the diffraction grating to substantially reach the dimensions of the optical mode of the optical fiber to enable a better coupling. An optical fiber 11 having one of its ends placed in front of diffraction grating 9 delivers a light beam 13 towards diffraction grating 9. When optical fiber 11 properly illuminates diffraction grating 9 (good alignment), a light beam (arrow 15) travels through the waveguide. It should be noted that the structure of FIG. 1 may also be used to transmit a light beam originating from an integrated optical circuit to optical fiber 11 via the core of waveguide 3.
For an optical circuit to operate properly and for the light to be coupled between an optical fiber and a submicronic waveguide of an integrated optical circuit, the optical fiber must be perfectly aligned with the coupling device associated therewith.
Several methods have been provided to perform this alignment. For example, the integrated optical circuit may be provided to deliver a light beam to the coupling device, and the alignment of the optical fiber is obtained when the amount of light that it conveys is maximum. It may also be provided to form a photodetector device in the integrated optical circuit to detect the position of the fiber enabling to convey the maximum light intensity towards the optical circuit.
However, such methods have the disadvantage of requiring the presence, in any integrated optical circuit, of elements dedicated to the alignment of the optical fibers, for example, illumination devices or photodetectors. Further, in the alignment, the integrated optical circuit must be in operation and thus requires to be powered. To overcome this disadvantage, it has been provided to modify the waveguides or the coupling gratings to be able to obtain an alignment signal during setting periods, but this complicates the manufacturing and risks disturbing the normal operation of the optical circuit.
U.S. Pat. No. 7,024,066 provides adding to an integrated optical circuit structure a specific grating intended for a positioning.
More specifically, as shown in top view in FIG. 2A and in cross-section view in FIG. 2B, this patent provides adding to an integrated optical circuit 41 comprising a functional grating 42 intended to introduce light into a submicronic waveguide 43 coupled with circuits elements, not shown, an additional grating 44 of Littrow grating type. As shown by the top view of FIG. 2B, the integrated optical circuit is formed on a substrate 46 and comprises submicronic waveguides 43 between two cladding layers 47 and 48. Littrow grating 44 is added on the upper surface of upper optical cladding 48. This US patent provides positioning an optical fiber 49 on Littrow grating 44 and the distance between gratings 44 and 42 is known by construction, displacing the optical fiber by the known distance separating positing grating 44 from functional grating 42.
However, this method, although it enables to position the fiber, does not enable to align it, that is, to adjust its orientation according to the optimal angle to be provided between the fiber and the functional grating. This method only enables to roughly adjust the alignment between an optical fiber and a grating associated with a functional waveguide. Indeed, for example for a 1,550-nm wavelength and a typical 10% manufacturing tolerance for the critical dimensions of the guide and of the grating, the coupling angle may vary by from 6° to 19° while the nominal value is 13°. The fiber/grating coupling rate falls by 1 dB (20%) for an angle variation of only 3 degrees. This is a first reason for which the use of a Littrow grating does not enable an alignment, but only a prepositioning: the Littrow grating will never have the same characteristics as the functional coupling gratings, this grating itself having variable characteristics. Further, even as concerns the positioning, the Littrow grating is manufactured on a different layer by masking operations different from those of functional gratings. The positioning tolerances are thus not strict and identical.